形式主语it的5种用法

▷例句1：Its a theory to which many economists subscribe, but in practice it often leaves railroads in the position of determining which companies will flourish and which will fail.

这句话中第二个it就是指代前面的theory。这是一个许多经济学家赞同的理论，但是在实际中，它往往会使得铁路处于一个决定哪些公司会兴旺，哪些公司会衰败的地位中。

第二种情况是非指代性it。

非指代性it是相对于作人称代词的it而言的。它们往往没有具体意义上的主语，它用于表示气候、温度、时间、地点、距离等。非指代性it常作seem, appear, look, happen, occur, follow等动词的主语，构成特定的句型。

▷ 例句：From an analysis of the hot-spot population it appears that the African plate is stationary and that it has not moved during the past 30 million years.

这句中it没有实际意义，为非指代性用法，that引导表语从句。从热点人口分析中可以看出，非洲板块是静止不动的，而且在过去的三百万年中也从未移动过。

it做先行词

it作形式主语或者形式宾语。It作形式主语时，往往放在句首，将真正的主语(动词不定式、动名词、从句)放在之后。其中it可作形式主语指代不定式，这时常见的谓语有：take, pay, make, cost, require, feel, annoy, need等。另外，It形式主语指代从句，而且主要形式为 It is+a.+than; It is+n.+that; It is+done+that; It+vi+that;

▷ 例如：It is difficult to the point of impossibility for the average reader under the age of forty to imagine a time when high-quality arts criticism could be found in most big-city newspaper.

it做形式宾语

是放在谓语动词与宾语补足语之间，指代不定式、动名词、从句等。后接it作形式宾语的谓语动词有：feel, consider, find, believe, make, take, imagine, suppose, think, deem, regard, count等。

引导强调句的it。

同学们所熟悉的强调句型中，它的基本形式就是“it+be的某种形式+that/who从句”。当被强调部分是人时须用who, 其他情况用that。

▷ 例句：Curbs on business-method claims would be a dramatic about-face, because it was the Federal Circuit itself that introduced such patents with its 1998 decision in the so-called State Street Bank case, approving a patent on a way of pooling mutual-fund assets.

在because从句中就运用了it was ….that的强调句型，强调部分为the Federal Circuit itself. 对商业方法专利申请的限制将会是一个巨大的改变，因为这些专利正是联邦巡回法院自己在1998年审理一个被称作“美国道富银行”的案件时提出的，判决中巡回法院批准了一项共同基金资产集资方法的专利。